Carboxylic Acids

Carboxylic acids (RCOOH) are the most acidic common organic functional group. The acidity arises from the resonance stabilization of the carboxylate ion (RCOO-): upon losing H+, the negative charge is equally delocalized over two equivalent C-O bonds (bond order 1.5 each). This stabilization makes carboxylic acids far more acidic than alcohols (where the alkoxide RO- has no such delocalization) and phenols (where charge delocalization is less effective).

Substituent Effects on Acidity: Electron-withdrawing groups (-I effect) stabilize the carboxylate ion further, increasing acidity. The order with halogen substitution: Cl₃CCOOH > Cl₂CHCOOH > ClCH₂COOH > CH₃COOH (CC(=O)O). The -I effect decreases with distance: 2-chlorobutanoic acid is more acidic than 3-chlorobutanoic acid. Electron-donating groups (+I effect of alkyl groups) destabilize the carboxylate ion, decreasing acidity: HCOOH (OC=O) > CH₃COOH > CH₃CH₂COOH > (CH₃)₂CHCOOH > (CH₃)₃CCOOH. Formic acid is stronger than acetic acid because the H in HCOOH has no +I effect, while the -CH₃ in CH₃COOH donates electrons, partially destabilizing the carboxylate.

Functional Group Acidity Ladder: carboxylic acid (pKa ~4-5) > phenol (~10) > alcohol (~16-18) > water (15.7) > terminal alkyne (~25). This order reflects decreasing conjugate base stability.

Preparation Methods: Oxidation of 1 degree alcohol or aldehyde (KMnO₄ or K₂Cr₂O₇ → RCOOH); Grignard reaction (RMgX + CO₂ →(dry ether) RCOOMgX →(H₃O+) RCOOH); hydrolysis of nitriles (RCN + H₃O+ → RCOOH + NH₄+); saponification (RCOOR' + NaOH → RCOONa + R'OH).

Key Reactions: The HVZ reaction (Hell-Volhard-Zelinsky) $\alpha$-halogenates carboxylic acids: RCOOH + X₂/red P → $\alpha$-halocarboxylic acid. This requires an $\alpha$-hydrogen — benzoic acid () and formic acid do NOT undergo HVZ. Decarboxylation with soda lime (NaOH + CaO): RCOONa + NaOH →(CaO, heat) R-H + Na₂CO₃. Kolbe electrolysis: 2RCOO- → R-R + 2CO₂ (at anode). Fischer esterification: RCOOH + R'OH →(H₂SO₄, heat) RCOOR' + H₂O (reversible, acid-catalyzed). SOCl₂ reaction: RCOOH + SOCl₂ → RCOCl + SO₂ + HCl (cleanest acyl chloride preparation — gaseous byproducts drive reaction forward). Reduction: RCOOH →(LiAlH₄) RCH₂OH. NaBH₄ does NOT reduce carboxylic acids — it is too weak a reducing agent. PCl₅ also converts RCOOH to RCOCl + POCl₃ + HCl.

The key testable concept is that NaBH₄ cannot reduce carboxylic acids (only LiAlH₄ can), and that the acidity of HCOOH exceeds CH₃COOH because the H atom has no +I effect to destabilize the carboxylate.